fix: cancel pending reconnection timeout on close()

Track the reconnection timeout ID and clear it when close() is called.
This prevents reconnection attempts after the transport is closed,
which was causing "Failed to reconnect SSE stream" errors on disconnect.

Adds test case to verify reconnection is cancelled after close().

Author: felixweinberger

src/client/streamableHttp.test.ts

@@ -921,6 +921,57 @@ describe('StreamableHTTPClientTransport', () => {
             expect(fetchMock.mock.calls[0][1]?.method).toBe('POST');
         });
 
+        it('should not attempt reconnection after close() is called', async () => {
+            // ARRANGE
+            transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
+                reconnectionOptions: {
+                    initialReconnectionDelay: 100,
+                    maxRetries: 3,
+                    maxReconnectionDelay: 1000,
+                    reconnectionDelayGrowFactor: 1
+                }
+            });
+
+            // Stream with priming event + notification (no response) that closes
+            // This triggers reconnection scheduling
+            const streamWithPriming = new ReadableStream({
+                start(controller) {
+                    controller.enqueue(
+                        new TextEncoder().encode('id: event-123\ndata: {"jsonrpc":"2.0","method":"notifications/test","params":{}}\n\n')
+                    );
+                    controller.close();
+                }
+            });
+
+            const fetchMock = global.fetch as Mock;
+
+            // POST request returns streaming response
+            fetchMock.mockResolvedValueOnce({
+                ok: true,
+                status: 200,
+                headers: new Headers({ 'content-type': 'text/event-stream' }),
+                body: streamWithPriming
+            });
+
+            // ACT
+            await transport.start();
+            await transport.send({ jsonrpc: '2.0', method: 'test', id: '1', params: {} });
+
+            // Wait a tick to let stream processing complete and schedule reconnection
+            await vi.advanceTimersByTimeAsync(10);
+
+            // Now close() - reconnection timeout is pending (scheduled for 100ms)
+            await transport.close();
+
+            // Advance past reconnection delay
+            await vi.advanceTimersByTimeAsync(200);
+
+            // ASSERT
+            // Only 1 call: the initial POST. No reconnection attempts after close().
+            expect(fetchMock).toHaveBeenCalledTimes(1);
+            expect(fetchMock.mock.calls[0][1]?.method).toBe('POST');
+        });
+
         it('should not throw JSON parse error on priming events with empty data', async () => {
             transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'));
 

src/client/streamableHttp.ts

@@ -136,6 +136,7 @@ export class StreamableHTTPClientTransport implements Transport {
     private _hasCompletedAuthFlow = false; // Circuit breaker: detect auth success followed by immediate 401
     private _lastUpscopingHeader?: string; // Track last upscoping header to prevent infinite upscoping.
     private _serverRetryMs?: number; // Server-provided retry delay from SSE retry field
+    private _reconnectionTimeout?: ReturnType<typeof setTimeout>;
 
     onclose?: () => void;
     onerror?: (error: Error) => void;
@@ -287,7 +288,7 @@ export class StreamableHTTPClientTransport implements Transport {
         const delay = this._getNextReconnectionDelay(attemptCount);
 
         // Schedule the reconnection
-        setTimeout(() => {
+        this._reconnectionTimeout = setTimeout(() => {
             // Use the last event ID to resume where we left off
             this._startOrAuthSse(options).catch(error => {
                 this.onerror?.(new Error(`Failed to reconnect SSE stream: ${error instanceof Error ? error.message : String(error)}`));
@@ -439,9 +440,11 @@ export class StreamableHTTPClientTransport implements Transport {
     }
 
     async close(): Promise<void> {
-        // Abort any pending requests
+        if (this._reconnectionTimeout) {
+            clearTimeout(this._reconnectionTimeout);
+            this._reconnectionTimeout = undefined;
+        }
         this._abortController?.abort();
-
         this.onclose?.();
     }